Active pixel image sensors, generally produced in CMOS (complementary metal oxide semiconductor) technology are routinely used in electronic imaging systems. In these sensors, each pixel comprises a photodiode, optionally a storage node for the charges photogenerated by the photodiode (“floating diffusion”) in the case of the use of a pinned photodiode, and a plurality of transistors (typically between 3 and 5 per pixel). A general introduction to these sensors is provided, for example, by the article by Abbas El Gamal and Helmu Rlyoukhy “CMOS Image Sensors”, IEEE Circuits & Devices Magazine, May-June 2005, pages 6 to 20.
It is known that active pixel image sensors can operate without the need to use a mechanical shutter. The shutter function is then obtained in a purely electronic manner, by an appropriate driving of the transistors of each pixel. The electronic shutter produced in this way can be of “rolling” or “global” type.
In the case of a rolling shutter, the integration periods of the different rows of pixels are phase-shifted relative to one another. The document WO2010066850 discloses an image sensor operating notably with a rolling shutter. In order to extend the dynamic range of the sensor, the reading of the charges of a pixel is conditioned therein by the proximity or not of the saturation of the storage node. The document FR2939950 discloses another sensor with a rolling shutter, in which an extension of the dynamic range is obtained by avoiding the saturation of the storage nodes in the case of a very bright image, while guaranteeing enough sensitivity for a dark image.
In the case of a global shutter, however, all the pixels integrate the incident light simultaneously, the reading of the pixels being done subsequently and row by row. Operation in rolling shutter mode is simpler to produce and makes it possible to maximize the integration time for a given image acquisition rate, and therefore the sensitivity of the sensor, but produces damaging artefacts (distortion of the scene, etc.) if the scene changes rapidly (moving object, sudden change in lighting conditions, etc.). Thus, some applications require a global shutter.
It is also known that active pixel image sensors are affected by different types of noise.
The document U.S.2007/045681 proposes a solution for reducing the noise linked to the dark current by performing charge transfers from the photodiode in a pulsed manner in order to limit this noise contribution. This noise is associated with the leakage current of the junction of the photodiode and originates from the phenomena of electron generation/recombination.
Among these different types of noise, the noise called “kTC”, or reset noise, can also be cited. Before acquiring an image, an active pixel is reset to an assumed known voltage (potential difference relative to ground); then it begins to accumulate photogenerated charges, which leads to a reduction of its voltage. If the reset voltage was perfectly known, the measurement of the voltage at the output of the pixel at the end of the integration would make it possible to determine the incident light intensity. In fact, the reset voltage varies unpredictably from one image to another because of the thermal fluctuations of the charge carriers. To eliminate this source of noise, it is necessary to sample the output voltage of the pixel twice—just after the reset and at the end of the integration period for a three-transistor (3T) pixel, or just after the reset and just after a charge transfer for a four-transistor (4T) or more pixel—and to subtract the first sample from the second. This technique is known as correlated double sampling (CDS). In its simplest form, correlated double sampling is not compatible with the use of a global shutter for matrix sensors that have a significant number of rows (of the order of ten or more). In this case, it is known practice to use a technique called “correlated quadruple sampling” (CQS), which consists in acquiring two images for each image, one in the absence of light (“black image” representative of the reset voltages of all the pixels) and the other after having illuminated the pixel for the desired integration time (“integration image” obtained after a global transfer of the charges in the matrix). The acquisition of the black and integration images is made possible by the fact that, in the pixels that are compatible with this technique, a transistor, called transfer transistor, makes it possible to isolate the source of the photogenerated charges (pinned photodiode) from a non-illuminated storage node. Both the black image and the integration image are obtained by difference between two voltage samples. The black image is the resultant of the difference between a reference voltage and a voltage acquired after the end of the reset of the storage node. The integration image is the resultant of the difference of a voltage acquired at the end of the integration time, after a transfer operation performed globally between the black image and the integration image, and of the same reference voltage. More specifically, first of all, row by row, the storage nodes are reset with the concomitant acquisition of the reset voltage which is a constant voltage level with little noise which serves as reference voltage; after a predefined time, the reset of the storage node is stopped and the voltage of the pixels representative of the noise sampled on the storage node is acquired again, the difference between the two acquisitions forming the black image. During this time, the photodiodes of all the pixels integrate electrical charges which, after the acquisition of the black image, are transferred globally to the corresponding storage nodes. To obtain the integration image, there is a first acquisition of the voltage at the terminals of the storage nodes followed, during the reset of said nodes, by a second voltage acquisition, to re-obtain the reference level, with little noise, and finally the subtraction of the two duly acquired voltages.
The black image obtained in this way essentially constitutes a measurement of the post-reset noise (random difference between the reset voltage and the voltage at the terminals of the storage node after reset), which noise itself affects the integration image, because the storage node is not reset between the two acquisitions. Consequently, this noise can be eliminated by subtraction of the two images. The article by B. Fowler et al. “A 5.5 Mpixel 100 Frames/sec Wide Dynamic Range Low Noise CMOS Image Sensor for Scientific Applications,” SPIE Proceedings 7536—Sensors, Cameras, and Systems for Industrial/Scientific Applications XI, describes an active pixel image sensor implementing the CQS technique combined with a global electronic shutter. One drawback of such a sensor is that the reading of the voltages of the pixels is necessarily destructive, because it is accompanied by a reset of the storage nodes in order to acquire the reference voltage. Now, a non-destructive read would be preferable in some applications, because it would make it possible to perform a plurality of readings of the pixels while continuing to integrate the photogenerated charges in order to stop the integration when the exposure level is optimal. Any risk of under or over-exposure would thus be avoided.